Often it is necessary to cool a working fluid, and it is known for this purpose to use a heat exchanger. Heat exchangers usually comprise one or more metallic tubes suspended between two tube plates. The working fluid to be cooled, which may for example be water or oil, flows through the tubes, whilst the coolant passes around and between those tubes, the working fluid giving up its latent heat to the tubes and thus to the coolant.
The effective surface area of a tube can be enlarged in order to increase the heat transfer, as by the addition of one or more annular fins in thermal contact with the outer surface of the tube. Such finned tubes are particularly useful if the coolant has a low viscosity, and if the coolant is a gas, such as air.
If the tubes are to withstand the internal pressure of the fluid to be cooled, the addition of the fins should not reduce or significantly reduce the tube bursting strength. If the fins are to increase the heat transfer they should not inhibit the flow of coolant, and preferably should encourage turbulent coolant flow.
The tubes to be used for heat exchangers should meet certain standards (in the UK for instance British Standard 2871 Part 3), these standards being relevant also for those tubes which are formed by extrusion to provide selected internal formations chosen to enhance internal turbulent flow i.e. to avoid laminar or stratified flow of the liquid to be cooled; desirably the tube finning should not reduce those respective standards e.g. of tube wall thickness and thus of strength, or of tube uniformity and fin engagement and thus of heat transfer to the fins.
The fins should be positioned on the tube so as to encourage maximum heat transfer to the coolant, which will not occur if the fin spacing is irregular, or if the fin angles are irregular (with an annular gap of varying axial length between adjacent fins).
If the tube walls need to be thinned to accept the fins, one or more of the tubes may burst in service and need to be plugged; if the fins are irregularly spaced and/or angled the performance of the heat exchanger will be reduced.
It is a known design criterion when constructing a matrix or array of finned tubes for a heat exchanger both to arrange the tubes as close as possible to each other (to reduce the size of the heat exchanger), and to have a maximum area of thermal transfer between the working fluid and the coolant (to maximise the possible heat exchange). When utilising tubes fitted with the known annular fins in such an array, the spacing between the tubes will be limited by the outer diameter of the fin(s); if as is usual the fins have circular outer peripheries there are areas between adjacent tubes which do not contribute to heat transfer, and a finning method and machine permitting fins to be fitted which can utilise these areas is desirable.
The performance of a heat exchanger in part depends upon the number of fins fitted to a tube, and to the total number of fins i.e. to the aggregate extended area available for heat exchange as well as to the positioning and disposition of those fins.